Class D amplifier (i.e., class D audio power amplifier) is a switching type power amplifier, which has the advantage of high efficiency and less heat generating, it has been widely applied to the field of consumer electronic products area, including intelligent TVs, intelligent cell phones and more. Referring to FIG. 1, commonly seen class D amplifiers (Inside the dashed box in FIG. 1, it is the internal structure of a class D amplifier,) include: Amplifier a (AMPa), Comparator a (CMPa), Comparator b (CMPb), Driver a and Driver b; their connection relationship is shown in FIG. 1. The operating principle of this class D amplifier is based on PWM (Pulse Width Modulation) mode: differential audio signals are amplified by the amplifier AMPa, then input into the comparators CMPa and CMPb in two routes separately, after comparing with the input triangle wave signals, two routes of PWM signals are achieved respectively, then the amplitudes of the two routes of PWM signals are amplified when passing through the driver a and driver b separately; finally, they are filtered and restored into audio signals, which drive the speaker to vibrate and give out voices.
When the amplitude of the input differential audio signals is overlarge, it could make the amplitude of the restored audio signals exceed the power rating of the speaker, or cause short circuit to the capacitor for input, Ca or Cb, resulting in making the restored audio signals a DC component, which could burn the speaker out. In order to avoid this kind of anomalies happen, the output amplitude of the class D amplifier needs to be restricted. Since the pulse width of the PWM signals, (i.e., the duty ratio) is proportional to the audio signal amplitude, therefore, it is possible to restrict the amplitude of the audio signals for output by limiting the duty ratio of the PWM signals.